It is known that a fixed aerodynamic surface of an aircraft (such as a wing, a horizontal stabilizer or a vertical stabilizer) is extended at the rear by at least one mobile flap (such as an aileron, an elevator, a rudder, etc.) able to form a mobile trailing-edge part for said aerodynamic surface.
It is also known that such mobile flaps are controlled by actuators mounted on the fixed aerodynamic surfaces. Of course, said actuators have to be designed for the forces they need to develop in order to move said flaps, it being necessary for these forces to overcome the aerodynamic forces applied to said flaps. Because these flaps can rotate, these aerodynamic forces generate, with respect to the hinge of said flaps, a resistive moment generally known as a “hinge moment”.
However, because said actuators are heavy components, they have to be specified just sufficient to do the job in order to limit the mass of the aircraft, and this means that the hinge moment has to be known with accuracy.
Furthermore, in a new aircraft development program, said actuators have to be defined very early on because they themselves undergo a lengthy development process. Now, hinge moments of an aircraft under development are not only difficult to predict with sufficient preciseness for the actuators to be specified optimally, but also vary greatly with changes in the geometry of the aircraft during the development process.
Hence, in practice, margins of safety are created in the specifying of said actuators so as to guarantee that the flaps will work in spite of the uncertainties in prediction and the possible variations in geometry. As a result, the actuators are always overspecified.
In an attempt to remedy the aforementioned disadvantages, proposals have already been made for external trailing-edge tabs to be arranged on control surfaces, these working by modifying the shape of said control surfaces. Such external tabs are detrimental to the aerodynamic performance of the aircraft because they increase the drag. In addition, the compensation they afford has constantly to be adjusted according to the phase of flight or the angle of the control surfaces. Furthermore, these external tabs are located at the trailing edge of the control surfaces and therefore add additional weight making said control surfaces sensitive to aerostatic flutter.
Elsewhere, document U.S. Pat. No. 2,630,988 already discloses a set of aerodynamic surfaces for aircraft, comprising at least:                one fixed aerodynamic surface delimited by two external walls which, at the rear, converge toward one another and between which a spar is positioned, said external walls and said spar delimiting a box section that is open to the rear and runs in the overall direction of the span of said fixed aerodynamic surface;        a mobile flap, the front part of which is articulated about an axis of rotation with respect to said fixed aerodynamic surface and which extends said external walls, thereby forming a mobile trailing-edge part for said fixed aerodynamic surface;        actuating means, housed in said box section and able to cause said mobile flap to rotate; and        a vane, positioned in that part of said box section that is free of said actuating means and on the opposite side of said axis of rotation to said flap, said vane rotating as one with said flap and running in the overall direction of the span of said fixed aerodynamic surface, said vane dividing said box section into two chambers at least substantially isolated from one another and each comprising one of said external walls.        
In a set of aerodynamic surfaces such as this, each chamber is in pressure-wise communication with the aerodynamic flow over the corresponding external wall of said box structure via the slot that there is by design between the rear part of said fixed aerodynamic surface and said mobile flap.
Thus, in theory, said vane is subjected to a pressure differential similar to the one applied to said flap and, because it is positioned on the opposite side of the axis of rotation therefrom, it generates an antagonistic moment opposing the hinge moment, thus decreasing the latter accordingly.
However, experience has shown that said pressure communication slot is unable, at said vane, to guarantee a pressure differential that can actually be used to generate such an antagonistic moment able optimally to oppose the hinge moment.